1. Field of the Invention
Embodiments of the present invention generally relate to plasma nitridation and, more particularly, to methods and apparatus for igniting and maintaining a plasma without causing ion bombardment damage to a device.
2. Description of the Related Art
Integrated circuits (ICs) are composed of many, e.g., millions, of devices such as transistors, capacitors, and resistors. Transistors, such as field effect transistors (FETs), typically include a source, a drain, and a gate stack. The gate stack generally includes a substrate, such as a silicon substrate, a gate dielectric, and a gate electrode, such as polycrystalline silicon, on the gate dielectric. The gate dielectric layer is formed of dielectric materials such as silicon dioxide (SiO2), or a high-k dielectric material having a dielectric constant greater than 4.0, such as SiON, SiN, hafnium oxide (HfO2), hafnium silicate (HfSiO2), hafnium silicon oxynitride (HfSiON), zirconium oxide (ZrO2), zirconium silicate (ZrSiO2), barium strontium titanate (BaSrTiO3, or BST), lead zirconate titanate (Pb(ZrTi)O3, or PZT), and the like. It should be noted, however, that the film stack may comprise layers formed of other materials.
As integrated circuit sizes and the sizes of the transistors thereon decrease, the gate drive current required to increase the speed of the transistor has increased. The drive current increases as the gate capacitance increases, and capacitance (C) is proportional to kA/d, wherein k is the dielectric constant of the gate, d is the dielectric thickness, and A is the area of the device. Decreasing the dielectric thickness and increasing the dielectric constant of the gate dielectric are methods of increasing the gate capacitance and the drive current.
Attempts have been made to reduce the thickness of SiO2 gate dielectrics below 20 Å. However, it has been found that the use of SiO2 gate dielectrics below 20 Å often results in undesirable effects on gate performance and durability. For example, boron from a boron doped gate electrode can penetrate through a thin SiO2 gate dielectric into the underlying silicon substrate. Also, there is typically an increase in gate leakage current, i.e., tunneling current, with thin dielectrics that increases the amount of power consumed by the gate. Thin SiO2 gate dielectrics may be susceptible to NMOS (n-channel metal oxide semiconductor) hot carrier degradation, in which high energy carriers traveling across the dielectric can damage or destroy the channel. Thin SiO2 gate dielectrics may also be susceptible to PMOS (p-channel metal oxide semiconductor) negative bias temperature instability (NBTI), wherein the threshold voltage or drive current drifts with operation of the gate.
A method of forming a dielectric layer suitable for use as the gate dielectric layer in a MOSFET (metal oxide semiconductor field effect transistor) includes nitridizing a thin silicon oxide film in a nitrogen-containing plasma. Increasing the net nitrogen content in the gate oxide to increase the dielectric constant is desirable for several reasons. For example, the bulk of the oxide dielectric may be lightly incorporated with nitrogen during the plasma nitridation process, which reduces the equivalent oxide thickness (EOT) over the starting oxide. This may result in a gate leakage reduction, due to tunneling during the operation of a FET, at the same EOT as the un-nitrided oxide dielectric. At the same time, such an increased nitrogen content may also reduce damage induced by Fowler-Nordheim (F-N) tunneling currents during subsequent processing operations, provided that the thickness of the dielectric is in the F-N range. Another benefit of increasing the net nitrogen content of the gate oxide is that the nitridized gate dielectric is more resistant to the problem of gate etch undercut, which in turn reduces defect states and current leakage at the gate edge.
The plasma of the plasma nitridation process can be created by various ionizing power sources, which may, for example, include an inductively-coupled power source, a capacitively-coupled power source, a surface wave power source, an electronic cyclotron resonance source (ECR source), magnetron or modified magnetron-type sources, or other ionizing sources that may be used to facilitate plasma generation in a processing chamber. A surface wave power source is a very high frequency (100 MHz to 10 GHz) plasma source, in which the gas collision frequency is much less than the electromagnetic wave frequency, such that the electromagnetic power is absorbed into the plasma by a “surface-wave” or “wave-heating” based energy transfer mechanism. Such a source typically includes a very high frequency power source, a wave guide connecting the power source to the chamber, a dielectric chamber wall and an arrangement of openings or slots adjacent to the dielectric wall in which the very high frequency power is coupled in to the chamber. A microwave ionization power source is a type of surface wave power source.
In order to ignite (i.e., start) a plasma in a processing chamber, there is typically a source of high voltage generating a strong electric field to accelerate some free electrons in a gas mixture contained within the chamber. Once such accelerated electrons gain enough energy and collide with the molecules of the gas mixture, the gas molecules will ionize and more electrons may be freed. These ionized gas molecules or atoms (ions) and electrons may then be further excited by an oscillating electric or magnetic field, such as that provided by a radio frequency (RF) generator or other type of excitation source, to create even more ionization in the gas mixture, thereby resulting in an avalanche effect of generating more and more charged particles. The plasma becomes sustaining when the energy input to the plasma from the excitation source equals the energy lost from the plasma.
Independent of the type of excitation source, there can be significant capacitive coupling from the source to the plasma, which creates a relatively large plasma potential (on the order of tens of volts) and a strong electric field normal to the surface of a substrate being processed in a plasma reactor. Since such a strong electric field causes ions or electrons to be accelerated with high kinetic energy towards the substrate surface, capacitive coupling may cause excessive bombardment of the surface of a substrate, such as the silicon dioxide layer, by nitrogen ions, for example, which can cause damage to the substrate surface. Therefore, inductively-coupled plasma is preferred since the excitation field is parallel to the substrate surface.
However, devices for maintaining inductively-coupled plasma typically cannot ignite the plasma because inductive coupling cannot generate high energy electrons to start the avalanche process described above. In such systems, either a secondary excitation source using capacitively-coupled RF is employed or the inductively-coupled RF source has a small capacitively-coupled element present to ignite the plasma, which may still lead to some damage to the substrate surface.
Accordingly, what are needed are a method and an apparatus for plasma ignition and maintenance that do not cause excessive damage of the substrate surface with energized ions or electrons.